The invention relates to a rapid-response braking device for braking a machine driven part.
One of the problems in high-speed stamping and bending machines consists of preventing or limiting, as far as possible, damage to the tool in the event of an excessive load which is placed on the tool. Thus it is known to use an overload clutch which separates the drive line of the motor from the tool if the scheduled load is exceeded (torque) or, with respect to a slipping clutch, which limits the torque transmitted by the drive shaft. Because of the momentum which the masses behind the overload clutch have, they continue to rotate when the overload clutch responds, even if it completely releases the masses from the motor in the drive line. The problem to be solved therefore consists of braking the continuously moving masses as quickly as possible so as to prevent them from causing any major damage. If a brake is provided between the motor and the overload clutch, the brake may brake and stop the motor, but not the section of the drive line which is released by the overload clutch and which connects with the tool. However, if a brake is provided behind the overload clutch, it is possible to brake the tool until it until it comes to a standstill and thus prevent damage, provided that the brake is fast enough. Unfortunately, the masses that have to be braked in the drive line behind the overload clutch (tools and gears) are generally larger than the masses which lie in the section of the drive line in front of the overload clutch. A brake provided in the drive line behind the overload clutch must therefore be provided with a respectively strong design, which has an adverse effect on the braking times. The braking time is made up of an excitation time which is characteristic for the design of the respective brake and of the pure braking time for dissipating the kinetic energy. Pneumatic and hydraulic brakes require higher excitation periods for driving valves and for building up the required pressure in the braking device than electromagnetic brakes. However, they can provide large braking moments. Fast control devices such as electromagnetic brakes allow for a very short excitation time of the brake coil, but provide a longer braking time. A short braking time is of importance in stamping and bending machines and, in particular, in machines for trimming and forming the legs of integrated circuits (IC) in which the tool is driven by a cam gear, i.e., the forward and backward movement is determined by a roller which rolls on the cam surface of an eccentric disk. A rotation of the eccentric disk is equivalent to one work cycle of the machine in which the tool is positioned, the position being controlled such that the tool is advanced for processing the workpiece (working stroke) and then retracted (idle stroke), and the workpiece is advanced or removed. It is known from experience that the control of the tool movement requires about half a turn of the eccentric disk. Thus, only half of a turn of the eccentric disk is available for conveying the workpiece, for positioning the workpiece, for controlling measurement for ensuring that the tool position is correct and for a possible switch-off of the machine if the workpiece position is incorrect. The extension or shortening of the brake angle thus finds entrance into the cycle time of the machine with a factor of up to 2, i.e., the extension of the brake time causes a considerable extension of the cycle time. In order to obtain high-speed machines, it is therefore decisive that the workpiece can be stopped very quickly so as to prevent the destruction of the tool in the event of a wrongly positioned workpiece. Until recently, this problem was addressed by improving the efficiency of the brakes, reducing the response times and placing the brake as closely as possible to the position to be protected at the end of the drive line. These measures were implemented so as to provide a reduction of the maximum occurring moment that is caused by the released mass moment of inertia. These attempts, however, are subject to certain limits, because high braking moments are required through larger braking devices whenever high mass moments of inertia are present.